reading a text file

[salmawy]

hi all i want to put a function that reads a text file and put it in textbox1
i used this code to read the last line

Textbox1.text = Io.file.readalllines("C:\c.txt").last

but i want it to read the third line
how can i do it
pls help
thx in advance

[jmcilhinney]

You could do ReadAllLines(...).Skip(2).First() but that is a bit inefficient, especially if it's a large file. You'd do better to create a StreamReader and call ReadLine three times, discarding the first two results.

[J-Deezy]

What about simply calling the third index?

Dim thirdLine As String = IO.File.ReadAllLines("C:\c.txt")(2)

I think that should work, or would that be inefficient?

[motil]

how about the old fusion way

Dim s As String() = System.IO.File.ReadAllLines("C:\c.txt)
        TextBox1.Text = s(2)

[jmcilhinney]

I think that should work, or would that be inefficient?

It would be inefficient for the same reason: calling ReadAllLines requires reading the entire file. There's no point reading every line if you only need to read the first three. It's not a big deal if the file is small but the bigger the file, the more inefficient it becomes.

[J-Deezy]

It would be inefficient for the same reason: calling ReadAllLines requires reading the entire file. There's no point reading every line if you only need to read the first three. It's not a big deal if the file is small but the bigger the file, the more inefficient it becomes.

Ah okay thanks, that makes sense. I'm not too savvy with the technical side of streamreader, but doesn't that load the whole stream before you start reading lines? Or is a stream much more efficient to load than all the lines?

[jmcilhinney]

Ah okay thanks, that makes sense. I'm not too savvy with the technical side of streamreader, but doesn't that load the whole stream before you start reading lines? Or is a stream much more efficient to load than all the lines?

A Stream isn't something you load, but rather something you open. The Stream isn't the data, but rather a conduit through which data can travel from a source to a destination.

Also, you aren't avoiding using a StreamReader by calling File.ReadAllText or File.ReadAllLines anyway. You don't have to use one directly, but how do you suppose those methods read the file? They use a StreamReader internally. Here's the implementation of File.ReadAllLines:

<SecuritySafeCritical> _
Public Shared Function ReadAllLines(ByVal path As String) As String()
    If (path Is Nothing) Then
        Throw New ArgumentNullException("path")
    End If
    If (path.Length = 0) Then
        Throw New ArgumentException(Environment.GetResourceString("Argument_EmptyPath"))
    End If
    Return File.InternalReadAllLines(path, Encoding.UTF8)
End Function

and here's the implementation of InternalReadAllLines that it calls:

Private Shared Function InternalReadAllLines(ByVal path As String, ByVal encoding As Encoding) As String()
    Dim list As New List(Of String)
    Using reader As StreamReader = New StreamReader(path, encoding)
        Dim str As String
        Do While (Not str = reader.ReadLine Is Nothing)
            list.Add(str)
        Loop
    End Using
    Return list.ToArray
End Function

As you can see, a StreamReader is created and ReadLine is called. It's just more convenient to make the one simple method call if you do want all the lines in the file but, if you don't, then it's more efficient to write a little bit more code and read only what you need.

[J-Deezy]

Ah okay thanks for the explanation John, I'll give you some rep for that

EDIT: apparently I can't rep you again?